The present invention relates to the organic light emitting devices for display applications and to methods for fabricating such devices.
Organic light-emitting devices (OLEDs) are typically manufactured as a sequence of layers deposited on top of each other to form a layer structure. The layer structure typically comprises a first electrode on a supporting substrate and one or more organic layers disposed between the first electrode and a second electrode. Light output is generated by charge injection into the organic material via the electrodes. The organic material emits photons on excitation by the injected charge. At least one of the electrodes is typically formed from a light transmissive material such as Indium Tin Oxide (ITO) or a thin metal to permit passage of light out of the device. Light transmissive materials should be understood to include both transparent and semi-transparent materials.
If the supporting substrate has a relatively low transparency, then it is desirable for light output to flow through the second electrode. An example of a relatively low transparency supporting substrate material is c-Si. Similarly, it is desirable for light output to flow through the second electrode if the aperture ratio is adversely affected by dimensions of underlying driving circuitry,
As demonstrated in U.S. Pat. No. 5,981,306 to Burrows et al. and U.S. Pat. No. 5,714,838 to Haight et al, this can be achieved via a thin semitransparent metallic layer or via a transparent conducting layer. To achieve sufficient transparency with a semitransparent metal, the layer thickness has to be very thin (typically  less than 20 nm). This causes an increase in sheet resistance and a significant voltage drop across the layer, especially at large display sizes. Hence, the power efficiency and the uniformity of light emission is adversely affected. A problem associated with deposition of a transparent conductor such as ITO on the organic layers is that such deposition may cause damage to the underlying layers thereby diminishing device performance considerably. The amount of damage increases with increasing deposition rates. The deposition rate is preferably made sufficiently low to reduce such damage to a tolerable amount. However, this reduces processing speed.
U.S. Pat. No. 5,965,979 to Friend et al. describes a manufacturing technique comprising laminating together two self-supporting components each having a light-emitting organic layer on top. A problem associated with this method is that the lamination does not provide an intimate contact to the organic layers at a microscopic level over large areas. Such contact is desirable in the interests of reproducible fabrication of efficient and uniform light-emitting devices.
In accordance with the present invention, there is now provided a method of making a light-emitting device comprising: forming a first component having a first substrate, the forming of the first component comprising locating a first electrode on the first substrate, locating an organic layer on the first electrode, and locating a light-transmissive second electrode on the organic layer; forming a second component having a light-transmissive second substrate, the forming of the second component comprising locating a light transmissive, electrically conductive layer on the second substrate; joining the first component and the second component with the second electrode of the first component facing the conductive layer of the second component; and, forming, an electrical contact between the second electrode of the first component and the electrically conductive layer of the second component.
The method preferably comprises locating spacers between the first component and the second component. At least one of the spacers may be formed on the electrically conductive layer of the second component. Similarly, at least one of the spacers may be formed on the first substrate. The or each spacer formed on the first substrate may be coated with the organic layer. Similarly, the or each spacer formed on the first substrate may be coated with the second electrode. It will be appreciated that at least some of the spacers may be formed on the second electrode. Conductive paths between electrically conductive layer and the second electrode may be established via the spacers. The locating of the first electrode may comprise patterning the first electrode on the first substrate. In preferred embodiments of the present invention, the locating of the spacers between the first component and the second component comprises positioning the spacers at sites remote from areas of the substrate occupied by the first electrode. In particularly preferred embodiments of the present invention, the joining of the first component and the second component comprises forming a peripheral seal between the first component and the second component and creating a vacuum within the peripheral seal.
Viewing the present invention from another aspect, there is now provided a light-emitting device comprising: a first component having a first substrate, a first electrode on the first substrate, an organic layer on the first electrode, and a light-transmissive second electrode on the organic layer; a second component having a light-transmissive second substrate and a light transmissive, electrically conductive layer on the second substrate; means for joining the first component and the second component with the second electrode of the first component facing the conductive layer of the second component; and, an electrical contact for electrically connecting the second electrode of the first component and the conductive layer of the second component.
The spacers may be distributed between the first component and the second component. At least one of the spacers may be integral to the first component. The or each spacer of the first component may be disposed on the first substrate. The or each spacer of the first component may be coated with the organic layer. Similarly, the or each spacer of the first component may be coated with the second electrode. Alternatively, the or each spacer of the first component is disposed on the second electrode. At least one of the spacers may be integral to the second component. In preferred embodiments of the present invention, the spacers are electrically conductive. The first component may comprise a plurality of organic layers disposed between the first electrode and the second electrode. The spacers are preferably located at sites remote from areas of the substrate occupied by the first electrode. The joining means preferably comprises a peripheral seal between the first component and the second component and a vacuum is disposed within the peripheral seal.
In a preferred embodiment of the present invention to be described shortly, there is provided an OLED for large area display applications and a method for making the same. The method is especially suitable for manufacture of such OLEDs on substrates which allow less than 80% transmission of the internally emitted light. The OLEDs described herein comprise two components. The first component has a substrate carrying an organic layer. A first electrode layer is disposed between the organic layer and substrate. A second electrode layer is disposed on the surface of the organic layer remote from the substrate. The second electrode layer comprises a thin semitransparent metal electrode layer ( less than 20 nm) in intimate electrical contact with an underlying organic layer to provide uniform charge injection in the interests of optimizing display output. The metal layer can be formed from any metal or combination of metals. The second component carries a highly electrically conductive layer. The first and second components are superimposed on each other with the conductive layer of the second component is overlying and in electrical contact with the second electrode of the first component. The conductive layer of the second component avoids significant voltage drop across the second electrode of the first component. The electrical contact between the conductive coating of the second component and the second electrode of the first component need not be in intimate electrical contact. Spacers are preferably provided to reduce the risk of mechanical damage when the first and second components of the OLED are brought together. Additionally, the spacers improve the mechanical stability of the OLED.
The conductive layer on the second component may be regarded as a part of the second electrode. And advantage of providing the second electrode in multiple parts is that each part can be optimized seperatly in terms of; for example, transparency, conductivity, low damage, injection efficiency, processing speed. Another advantage is that the conductive contact between the parts does not have to be intimate over the entire display area. This is an improvement over conventional techniques, such as that described in the aforementioned Friend et al. reference, in which the lamination of two organic compounds described therein clearly demands an intimate conductive contact. In embodiments of the present invention, the lateral conductivity of the parts is sufficient to bridge a distance of several micrometers between periodic contacts. Embodiments of the present inventions are therefore much more tolerant to mechanical strain.
In another preferred embodiment of the present invention to be described shortly, there is provided a light-emitting device comprising: a substrate having a plurality vias formed therein; a first electrode on the substrate, an organic layer on the first electrode; a light-transmissive second electrode on the organic layer; and an electrically conductive layer communicating with second electrode through the vias. The conductive layer may be embedded in the substrate. Alternatively, the conductive may be disposed on the side of the substrate remote from the first electrode.